“Contempt prior to investigation is what enslaves a mind to Ignorance.”

Commonly attributed to Spencer, information provided in The Quote Verifier: Who Said What, Where, and When (2006) by Ralph Keyes and The Survival of a Fitting Quotation. (2005) by Michael StGeorge http://anonpress.org/spencer/, indicates the attribution may have originated with the book [w:The_Big_Book_(Alcoholics_Anonymous)|Alcoholics Anonymous]]. It was used exactly as written above in the personal stories section of the first edition in 1939 and in 'Appendix II: Spiritual Experience' of all subsequent editions.
: "Contempt prior to examination" was a phrase used by William Paley, the 18th-century English Christian apologist. In A View of the Evidences of Christianity (1794), he wrote:
::The infidelity of the Gentile world, and that more especially of men of rank and learning in it, is resolved into a principle which, in my judgment, will account for the inefficacy of any argument, or any evidence whatever, viz. contempt prior to examination.
:Paley's characterization of non-believers was later modified and used by other religious authors who uniformly attributed their words to Paley. In Anglo-Israel or, The British Nation: The lost Tribes of Israel (1879), Rev. William H. Poole may have been the first to render the quotation in its more familiar and enduring form:
::There is a principle which is a bar against all information, which is proof against all argument, and which cannot fail to keep a man in everlasting ignorance. This principle is, contempt prior to examination.
:Various authors following Rev. Poole would offer new iterations of the quotation into the early decades of the 20th century. Most of these credited William Paley, but by the early 1930s the first obscure publications to falsely attribute this quote to Spencer emerged. Its usage for decades since as a maxim in Alcoholics Anonymous and the twelve-step recovery community has popularized its erroneous association with Herbert Spencer.
Misattributed

Source: Thorns (1967), Chapter 21, “And Southward Aye We Fled” (p. 106)

Notes on Language and Style (1929)

In 'The Languedocian Scorpion'

#125
1900s, Maxims for Revolutionists (1903)

Introduction
An Enquiry into the Obligations of Christians (1792)
Context: In one period the grossest ignorance and barbarism prevailed in the world; and afterwards, in a more enlightened age, the most daring infidelity, and contempt of God; so that the world which was once over-run with ignorance, now by wisdom knew not God, but changed the glory of the incorruptible God as much as in the most barbarous ages, into an image made like to corruptible man, and to birds, and four-footed beasts, and creeping things. Nay, as they increased in science and politeness, they ran into more abundant and extravagant idolatries.

Refuge Recovery (2014)

Untersuchen was ist, und nicht was behagt
Der Versuch als Vermittler von Objekt und Subjekt (The Attempt as Mediator of Object and Subject) (1792)

The Ultimate Quotable Einstein by Alice Calaprice lists this as "probably not by Einstein". However, this post from quoteinvestigator.com http://quoteinvestigator.com/2014/04/29/common-sense/ traces it to a reasonably plausible source: the second part of a three-part series by Lincoln Barrett (former editor of 'Life' magazine) titled "The Universe and Dr. Einstein" in Harper's Magazine, from May 1948, in which Barrett wrote "But as Einstein has pointed out, common sense is actually nothing more than a deposit of prejudices laid down in the mind prior to the age of eighteen." Since he didn't put the statement in quotes it could be a paraphrase, and "as Einstein has pointed out" makes it unclear whether Einstein said this personally to Barrett or Barrett was recalling a quote of Einstein's he'd seen elsewhere. In any case, the interview was republished in a book of the same title, and Einstein wrote a foreword which praised Barrett's work on the book, so it's likely he read the quote about common sense and at least had no objection to it, whether or not he recalled making the specific comment.
Unsourced variant: Common sense is the collection of prejudices acquired by age eighteen.
Disputed